1. Field of the Invention
The present invention relates to a lamp lighting apparatus to drive a plurality of discharge lamps (multiple discharge lamp lighting apparatus), and particularly to a multiple discharge lamp lighting apparatus to drive cold cathode lamps or the like used as a light source of a backlight system for a liquid crystal display device.
2. Description of the Related Art
A discharge lamp, for example, a cold cathode lamp, is extensively used as a light source of a backlight system for a liquid crystal display (LCD) device, and such a discharge lamp is usually AC driven by a discharge lamp lighting apparatus provided with an inverter. Recently, as an LCD device becomes larger in size for a higher brightness, a multiple lamp backlight device adapted to drive a plurality of discharge lamps is more and more used as a lighting source for an LCD device.
Generally, a high voltage is required for driving a discharge lamp, and therefore a discharge lamp lighting apparatus usually includes an inverter transformer to generate a high voltage at the secondary side. An inverter means to generate a high frequency voltage is provided at the primary side of the inverter transformer, while a discharge lamp having a negative resistance characteristic, and a so-called ballast element, such as a ballast capacitor, to stabilize the lamp current of the discharge lamp are provided at the secondary side of the inverter transformer. In a conventional multiple discharge lamp lighting apparatus to drive a plurality of discharge lamps, a ballast capacitor is connected to each of the discharge lamps (refer to, for example, Patent Document 1).
A multiple discharge lamp lighting apparatus is required to provide a uniform lamp current for all discharge lamps in order to achieve a uniform brightness among all the discharge lamps. However, if an individual ballast capacitor is connected to each of the plurality of discharge lamps, the characteristic variation among the individual ballast capacitors may possibly cause lamp current variation among the discharge lamps. To cope with this variation problem, a multiple discharge lamp lighting apparatus is disclosed which includes a circuitry in which a balance coil is provided at the secondary side of an inverter thereby uniformizing the lamp currents of all the discharge lamps (refer to, for example, Patent Document 2). Also, another multiple discharge lamp lighting apparatus is disclosed which includes a circuitry in which electric power is supplied from a low voltage constant current source provided at the primary side of an inverter thereby eliminating requirement of a ballast capacitor (refer to, for example, Patent Document 3), and this circuitry is expected to have a certain effect on achieving a uniform lamp current for the plurality of discharge lamps.    Patent Document 1: Japanese Patent Application Laid-Open No. 2002-175891    Patent Document 2: Japanese Patent Application Laid-Open No. H7-45393    Patent Document 3: Japanese Patent No. 3256992